Considerable research has been conducted recently in the area of producing alkenes for use as industrial raw materials. Among the many uses of such commodity chemicals include plastic and fibers for consumption in packaging, transportation and construction industries. Of particular interest are areas of research focusing on production of alkenes, such as ethylene, which is consumed principally in the manufacture of polyethylene, and substituted alkenes, such as ethylene dichloride and vinyl chloride. Ethylene is also employed in the production of ethylene oxide, ethyl benzene, ethylene dichloride, ethylene-propylene elastomers and vinyl acetate.
The primary sources of alkenes, such as ethylene, include: steam cracking of organics, such as gas oils; off-gas from fluid catalytic cracking (FCC) in oil refineries, catalytic dehydration of alcohols; and recovery from coal-derived synthesis gas. However, the worldwide demand for alkenes is extraordinary: the short fall in worldwide supply of ethylene alone was estimated in 1991 to be about 2.3 million tons, as determined by the Chemical Economics Handbook, SRI International (1992). Further, known methods for producing alkenes have significant drawbacks. For example, organic steam cracking, which accounts for about 100% of ethylene production in the U.S., is a mature technology which is highly sensitive to process variables, such as cracking severity, residence time and hydrocarbon partial pressure, as well as plant economics and price fluctuation. Other methods, such as alkene cracking over a solid support, can cause "coking up," which requires frequent burnout of the solid support to continue processing. In addition, such processes are facing increasing environmental regulatory pressure to control systemic problems, such as leaks and failure from related equipment and safety concerns associated with alkene cracking.
Other listed production methods have even greater limitations. The availability of FCC off-gas, for example, generally prohibits its use as an economically viable feed stock. Catalytic dehydration of alcohols is effectively limited to certain countries that have large amounts of readily available fermentation raw material. Also, known methods for production of alkenes from other sources, such as coal and coal-derived naphtha and methanol are, at best, only marginally commercially viable.
Further, Government pressure to recycle plastic materials is increasing due to the more than two billion pounds of packaging plastics that are introduced into the market each year. However, the potential significance of recycling plastic into feedstocks, syncrude oils or monomers is hampered by costs that are typically higher than those of other ways of dealing with plastic waste. Incineration with energy recovery is the method that industry generally prefers. Although recycling postconsumer waste is highly desirable, typical plastic containing wastes do not meet the criteria for feasibility of existing processes. For example, collection methods are variable and the waste often needs to be cleaned or preprocessed in some fashion that adds to the recycling cost.
Using chemolysis methods of depolymerizing single condensation polymers, such as polyethylene terephthalate (PET), nylon and polymethyl methacrylates (PMMA), is theoretically possible, but the difficulty is in handling mixed plastics. Bulk thermoplastics that are present in mixed plastics can generally be broken down only by thermal cracking to naphthalene feed stock. Other technologies include gasification of plastics with coal sources, oxygen and steam to produce synthesis gas. Coal liquefaction methods can also produce a syncrude oil from mixed plastics.
Therefore, a need exists for an improved method of producing alkenes which significantly reduces or eliminates the above-mentioned problems.